Microfluidic devices were designed to perform on micromoles of biological macromolecules and viruses the search and the optimization of crystallization conditions by counter-diffusion, as well as the on-chip analysis of crystals by X-ray diffraction. Chips composed of microchannels were fabricated in poly-dimethylsiloxane (PDMS), poly-methyl-methacrylate (PMMA) and cyclo-olefin-copolymer (COC) by three distinct methods, namely replica casting, laser ablation and hot embossing. The geometry of the channels was chosen to ensure that crystallization occurs in a convection-free environment. The transparency of the materials is compatible with crystal growth monitoring by optical microscopy. The quality of the protein 3D structures derived from on-chip crystal analysis by X-ray diffraction using a synchrotron radiation was used to identify the most appropriate polymers. Altogether the results demonstrate that for a novel biomolecule, all steps from the initial search of crystallization conditions to X-ray diffraction data collection for 3D structure determination can be performed in a single chip.

This review presents the goals and principles of microfluidic technologies applied to the crystallization of biological macromolecules. A comparison of the devices that are available commercially or described in the literature summarizes the current state-of-the-art in microfluidics. A novel chip based on the counter-diffusion of solute molecules playing the role of crystallization agents is described. Inside the microfluidic channels composing the chip mass transport essentially occurs by diffusion. The chip is made of a rigid polymer that is impermeable to gases and compatible with crystal examination and monitoring in polarized light. The selected material is also transparent to X-rays, and three-dimensional protein structures can be determined from crystals contained inside this device using X-ray diffraction data collected on a synchrotron source. The outstanding quality of the electron-density maps demonstrates that on-chip crystal analysis is feasible. The replacement of conventional crystallization setups by inexpensive microfluidic chips for screening best crystallization agents and automated crystal diffraction analysis is discussed.